1. Field of the Invention
The present invention relates to a magneto-optical recording medium and a magneto-optical record/reproducing method, and more particularly to a magneto-optical recording medium provided at least with first and second magnetic layers which are mutually exchange-coupled, and a magneto-optical record/reproducing method utilizing said magneto-optical recording medium.
2. Related Background Art
Magnet-optical recording medium, on which the recording and reproduction are conducted by laser beam irradiation, are currently attracting attention as a recording medium capable of high density recording. For such a magneto-optical recording medium, there is currently employed a structure with at least two laminated magnetic layers with mutual magnetic coupling. In such a magneto-optical recording medium, various trials are being conducted for increasing the linear recording density and the track density, thereby enabling recording and reproduction with a higher recording density.
The minimum size of the irradiation area of a laser beam cannot be easily reduced, as it is defined by the diffraction limit of the laser beam and automatically determined by the wavelength .lambda. of the laser beam and the numerical aperture of the optical system. On the other hand, since the recording process on the magneto-optical recording medium is basically a thermal process, a recording with a cycle length (pit length) less than the diffraction limit is achieved, by making the heated area smaller than the laser irradiated area. However, as the reproduction process is based on the magneto-optical effect, it is difficult, normally, to reproduce a signal of a cycle length smaller than the laser irradiation area. It is therefore important, for increasing the recording density of the magneto-optical recording medium, to enable reproduction of the signal of a smaller cycle length. Therefore, attempts have been made for increasing the recording density by improvements in the configuration of the magneto-optical recording medium or in the reproducing method.
As an example, the Japanese Patent Laid-open Application No. 3-93058 discloses a method of employing a multi-layered film including a reproduction layer and a record holding layer which are magnetically coupled, recording signals in said record holding layer while maintaining said reproduction layer in the erased state by aligning the magnetizing direction in advance, and then heating the reproduction layer with the laser beam irradiation, thereby transferring the magnetization of the record holding layer to the heated area of the reproduction layer and reproducing the signals from said heated area by means of the magneto-optical effect. In this method, in comparison with the area irradiated with the laser beam in the reproducing optical system, the area of the reproduction layer, heated to the temperature required for the transfer of magnetization, is smaller. Also, as the unheated area is in the erased state, the interference between the codes is reduced at the signal reproduction, so that the reproduction of signals of a cycle length less than the diffraction limit of the laser beam is rendered possible.
However, in the above-explained reproduction method in which the reproduction layer is maintained in the erased state in advance and the magnetization of the record holding layer is transferred to the reproduction layer under the laser beam irradiation, there is required an operation of applying an external magnetic field to the reproduction layer in a location different from the location of laser beam irradiation, in order to set the entire reproduction layer in the erased state prior to the reproducing operation, so that the record/reproducing apparatus becomes inevitably complex.
Also in case of effecting the multi-beam direct verification (effecting the recording operation and the verifying operation for reading the obtained record for confirming the proper recording operation, in collective manner with plural laser beams) with a single optical system for the purpose of increasing the recording speed, a verifying laser beam is to be directed to an area immediately after recording, but it is not possible to shift the reproduction layer of said area to the erased state within the interval from the recording operation to the succeeding verifying operation. For this reason, at said verifying operation, it is not possible to effect the above-explained method of reading the signal only from the area heated by laser beam irradiation.